<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <link rel="stylesheet" href="css/bootstrap.css">
    <style>
        [class^="col"]{
            height: 40px;
            border:1px solid #333;
        }
    </style>
</head>
<body>
    <!-- 
        pc   4个  992px  width> 992px  col-md
        ipad  3个      768px<width<992px  col-sm
        iphone 2个     col-xs
     -->
    <div class="container">
         <div class="row">
            <div class="col-md-3 col-sm-4 col-xs-6">

            </div>
            <div class="col-md-3 col-sm-4 col-xs-6">

            </div>
            <div class="col-md-3 col-sm-4 col-xs-6">

            </div>
            <div class="col-md-3 col-sm-4 col-xs-6">

            </div>
         </div>
    </div>
    <script src="https://cdn.jsdelivr.net/npm/jquery@1.12.4/dist/jquery.min.js"></script>
    <script src="https://cdn.jsdelivr.net/npm/bootstrap@3.3.7/dist/js/bootstrap.min.js"></script>
</body>
</html>